A first feature of the present invention deals with the variation of the current sense ratio variation which is encountered in trench type current sense MOSFETs.
In a current sensing MOSFET, a small fraction of the total active area of a power MOSFET is employed to sense the current through an independent source terminal. The current sense ratio is the ratio of current conducted through the main FET and the sense die, which is basically determined by the ratio of effective current conduction areas. In many applications, a sense ratio as high as 5,000-15,000 is required, which needs a proportionally small active area for conducting the sense current. Compared to the small active area (e.g., 20 μm2) in a sense die, the peripheral region of the active area is relatively large (e.g., 40% of the active area). Because the amount of current conducted in the peripheral region is subject to variation due to a number of processing or manufacturing steps, it is difficult to produce current sense MOSFETs with low sense ratio variation. Moreover, reducing the cell pitch poses increased challenges in controlling sense ratio variation.
It would be very desirable to provide a trench structure and process that can reduce the variability in the current sense ratio in die of different wafers and even of die from a common wafer.
A second problem dealt with by the present invention is the reduced ability of the small current sense die (or die areas) to withstand electrostatic discharge (ESD) destruction.
Electrostatic discharge destruction of a MOSFET occurs when the voltage across two pins induced by static charge is high enough to damage the dielectric thin film of the device. For a certain amount of charge superimposed across a capacitor, the lower the capacitance value is, the higher the voltage across the capacitor will be. As the active area of a MOSFET decreases, the capacitance decreases, and the sensitivity of a MOSFET to ESD increases.
In a current sensing MOSFET, a small fraction of the total active area is employed to sense the current through an independent source terminal. In a high sense ratio current sensing MOSFET (e.g. sense ratio of 10,000 or more), the active area of a sense die is proportionally smaller than that of the main FET, and the gate-to-source sense pin capacitance of the sense die is also proportionally smaller (e.g. approximately 10,000 times smaller) than that of the main FET. (Note that the source of the sense die is the current sense pin.) The small gate-to-source (sense pin) capacitance makes a current sense die prone to ESD damage. The yield of a current sensing MOSFET wafers is often decreased due to ESD damage. ESD destruction of the device is also a reliability risk.
In order to protect a current sensing MOSFET from ESD destruction, a Zener diode across gate-to-source (sense pin) is often integrated in the die at the cost of increased processing steps.
It would be desirable to improve the ESD withstand capability of current sensing MOSFETs without added process costs.